1. Field of the Invention
The present invention relates to a raster left-and-right distortions correcting apparatus for correcting distortions of scanning lines such as left and right pin-cushion distortions which appear in a CRT (cathode ray tube) display device.
2. Description of the Related Art
FIG. 29 is a view showing the structure of a conventional raster left-and-right pin-cushion distortions correcting apparatus which has been disclosed in Japanese Patent Application Publication No. 3-74496. There are shown in FIG. 29, a vertical deflection coil 1, a vertical deflection output circuit 2 for supplying a vertical deflection current I.sub.V to the vertical deflection coil 1, a horizontal deflection coil 3, a horizontal deflection output circuit 4 for supplying a horizontal deflection current I.sub.H to the horizontal deflection coil 3, a waveform shaping circuit 5 for generating a parabola waveform voltage V.sub.P having a vertical deflection period in response to the vertical deflection current I.sub.V supplied from the vertical deflection output circuit 2, and a modulator 6 for amplitude-modulating the horizontal deflection current I.sub.H flowing in the horizontal deflection coil 3 based on the parabola waveform voltage V.sub.P.
FIG. 30 is a view specifically showing the configuration of the waveform shaping circuit 5. The waveform shaping circuit 5 is made up of a capacitor C which integrates a sawtooth-like vertical deflection current I.sub.V having a period T and an amplitude a, which is supplied from a vertical deflection output circuit 2 via a vertical deflection coil 1 and generates a parabola waveform voltage V.sub.P.
FIG. 31 is a view for describing left and right distortions of scanning lines on the screen of a CRT, which are corrected by the raster left-and-right pin-cushion distortions correcting apparatus. When horizontal and vertical deflection coils in a normal color CRT are normally so designed that a convergence miss is minimized, the shape of a raster is not represented in the form of a proper rectangle indicated by dotted lines as shown in FIG. 31. Thus, so-called left and right pin-cushion distortions appear in which the left and right sides of the raster are inwardly bent in bow form as indicated by solid lines.
The operation of the raster left-and-right pin-cushion distortions correcting apparatus will now be described below.
When a vertical synchronizing signal is supplied to the vertical deflection output circuit 2, the vertical deflection output circuit 2 supplies the vertical deflection current I.sub.V having the sawtooth waveform to each of the vertical deflection coil 1 and the waveform shaping circuit 5. Thus, the waveform shaping circuit 5 integrates the vertical deflection current I.sub.V using the capacitor C shown in FIG. 30 so as to generate the parabola waveform voltage V.sub.P and supplies it to the modulator 6.
When, on the other hand, a horizontal synchronizing signal is supplied to the horizontal deflection output circuit 4, the horizontal deflection output circuit 4 supplies the horizontal deflection current I.sub.H having a sawtooth waveform of a horizontal period to the horizontal deflection coil 3. The horizontal deflection current I.sub.H is amplitude-modulated based on the parabola waveform voltage V.sub.P supplied to the modulator 6. As a result, the current I.sub.H modulated based on the parabola waveform voltage V.sub.P having a vertical deflection period flows in the horizontal deflection coil 3. Thus, the length of each scanning line on the raster is corrected so that the left and right pin-cushion distortions are corrected.
Described specifically, assuming that the vertical deflection period, the amplitude of the sawtooth waveform and the time are represented as T, a and t respectively, an instantaneous value i of the vertical deflection current I.sub.V flowing in the vertical deflection coil 1 is represented as follows: EQU i=(a/T).multidot.t (1)
Thus, a voltage V.sub.C which appears across the capacitor C is represented as follows: EQU V.sub.C =(1/C).multidot..intg.idt ={a/(2CT)}.multidot.t2 (2)
A zero peak value V.sub.P of the voltage V.sub.C is given as a voltage value at the time of t=T/2. Therefore, if the voltage V.sub.C at the time of t=T/2 is represented as [V.sub.C ], then the V.sub.P is given as follows: EQU V.sub.P =[V.sub.C ]=aT/(8C) (3)
The parabola wave in which the voltage V.sub.P is regarded as the peak value, varies in proportion to the vertical deflection period T or in inverse proportion to a vertical deflection frequency. As a result, the horizontal deflection current I.sub.H is modulated so as to reach the maximum in the vicinity of the center of the vertical deflection period T to thereby correct the left and right pin-cushion distortions bent in bow form.
Since the conventional raster left-and-right pin-cushion distortions correcting apparatus is constructed as described above, only the typical left and right pin-cushion distortions indicated by the solid lines in FIG. 31 can be corrected. However, the CRT actually includes factors of distortions which cannot be removed by the parabola waveform due to a variation in its structure and the like. Therefore, the conventional raster left-and-right pin-cushion distortions correcting apparatus is accompanied by a problem that the left and right distortions having factors other than the pincushion distortions cannot be completely removed.
A method of generating a parabola waveform voltage having a vertical deflection period is considered as an alternative to the circuit shown in FIG. 30. However, even in either case, the amplitude of a vertical deflection frequency varies so as to reach the maximum in the vicinity of the center of the vertical deflection period so long as an integration process is effected, thus causing problems that other left and right distortions cannot be corrected by using such a method as it is and even when typical pin-cushion distortions are corrected, they need to be readjusted each time a vertical deflection frequency to be handled is changed.